Electronic controlled power switching apparatus provide useful alternative to conventional rocker-type switches so that power switching, for example, switching of electrical appliances and lighting apparatus, can be done by remote or wireless controlled switching, non-contact switching, touch switching or other intelligent or more sophisticated switching methods.
Electronic controlled power switching apparatus are typically controlled and operated by electronic control means which comprise, for example, a microprocessor. The electronic control means operate the making or breaking of an electronic controllable power switching device inside the power switching apparatus upon receipt of a control command or upon fulfilment of certain prescribed conditions. In order to provide an economical and relatively maintenance-free electronic controlled power switching apparatus, it is highly desirable that the power for operating the control and other peripheral circuitry of the switching apparatus is obtained from the alternate current (AC) power source to which the power switching apparatus is connected so that batteries are not essential to its operation. In many power distribution wiring networks, for example, networks at home or offices, extra power supply lines are not readily available to cater for the operating power requirements of an electronic controlled power switching apparatus.
To obviate the need of additional wirings for supplying operating power to the electronic control circuitry, electronic controlled power switching apparatus with power coupling means, which comprise a current transformer and a voltage transforming circuitry (such as voltage clamping circuitry) for coupling operating power from the AC power source to the rectifying circuitry of the power switching apparatus respectively during the “ON” and “OFF” states, are known. The primary windings of the current transformer are usually connected in series with and between the AC power source and the load. Because of the serial connection, a high current rating current transformer, typically with windings of copper wire of a large core diameter, will be required for a high load current rating, since copper wire of a large cross-section is desirable for reducing adverse heat generation. On the other hand, for an electronic controlled power switch with a low load current rating, a current transformer with a large number of turns in the primary winding is necessary to maintain sufficient operating power to the control and peripheral circuitry. Hence, an electronic controlled power switch with a large current rating range using conventional design would mean that a current transformer with primary windings comprising a large number of turns of a large core copper wire would be necessary.
This dual requirement has been a major obstacle preventing electronic controlled power switching apparatus with a large operating current range from being used in practical applications, especially in applications in which a compact design is desired. Hence, it is not surprising that electronic controlled power switching apparatus adapted for mounting in wall-sockets rarely provides a current rating range of between 0 to 10 AX (amperes) due to the size of the current transformer required for such a load current rating range. Hence, it is highly desirable if there can be provided electronic controlled power switching apparatus with a relatively large current rating range while maintaining a reasonably compact size to enhance the practical utility of electronic controlled power switching apparatus.
For power switching apparatus with a relatively large current rating, electronic controlled relays with mechanical means for making and breaking the electrical connection between the current conducting terminals of the relays are commonly used. However, the problem of electric arcing during the breaking of the mechano-electrical contacts of the relays, especially in a highly inductive circuit, may cause premature wear-out or even failure of the relays, for example, due to carbonization of the contacts. Furthermore, during the making and breaking transitions, bouncing may occur at the relay contacts and causes arcing. In order to factor in the adverse effect of electrical arcing, relays of a higher power handling rating (and therefore a significantly larger size) are frequently used to provide additional safety margins. For example, a 4 kVA rated relay at 250 Vac voltage rating may be required for a 250V 5 A application at 0.4 power factor to cater for possible arcing due to back EMF during breaking of the contacts, even though the maximum switching voltage which may appear across the current conducting terminals under normal operating conditions would be small.
Likewise, when used in a capacitive circuit, the in-rush current during the making of the relay contacts can be very high and the current rating of a relay may far exceed the “carry current” of a relay, which is the steady state current flowing through the current conducting terminals of the relay under normal operating conditions in order to provide additional safety margin during the making of the current conducting terminals. For example, in a highly capacitive circuit, the in-rush current can be as high as 1000 A even though the steady state operating current may be as low as 10 A.
Hence, it will be beneficial if there can be provided electronic controlled power switching apparatus with relays and with means to alleviate the need of relays of excessive power rating to provide for an adequate safety margin which will inevitably result in a larger apparatus size.